According to our and other ethnobotanic studies (Walker, R., 1951. Une Nouvelle Légumineuse du Gabon servant à narcotiser le poisson. Rev. Bot. Appl. 31, 327; Walker, R., Sillans, R., 1961. Les plantes utiles du Gabon. Encyclopédie Biologique. P. Chevalier, Paris; Halle, N., 1970. Flore du Gabon 17, Famille des Rubiacées. Museum National d'Histoire Naturelle, Paris; Mounzeo, H., et al., 1997. Quelques plantes utilisées comme poisons de pêches chez les Punu du Gabon. Le Flamboyant 44. Décembre, Bulletin de Liaison des membres du réseau 'Arbres Tropicaux'), Tephrosia vogelii and Justicia extensa are two plants whose leaves are particularly used for the catching of fish in Gabonese rivers. The leaf extracts of those plants have been tested on Tilapia nilotica in order to observe their toxicity. At a given dose, the small fish are the first to be poisoned. This toxicity is more important for J. extensa and increased in a dose-dependent manner. After boiling for 90 min, those leaf extracts and rotenone (10(-6) M) taken as a control retain their toxicity at high dose (625 mg/l), although the latency period is higher. With the same temperature condition, at weak doses (37.5 and 62.5 mg/l), T. vogelii loses its toxicity, whereas J. extensa preserves it at 62.5 mg/l. As shown in our results, the fact that the extracts preserve their toxicity at high dose after boiling requires particular attention be given to the doses used for fishing and to the type of plants used.